The present invention relates to a timing belt tensioner for a motor vehicle engine.
It is known in the belt tensioner art to provide a frictionally damped timing belt tensioner. One such tensioner is disclosed in U.S. Pat. No. 4,824,421 and reissue U.S. Pat. No. RE. 34,543, which are hereby incorporated by reference in full. These patents disclose and teach a tensioner in which the belt load force applied by the belt to the tensioner is translated into force of engagement between friction surfaces which cooperate to dampen pivotal movement of the tensioner arm. The frictional force between these surfaces, which is a function of (among other things) the amount of belt load force applied by the belt, is the primary (essentially total) means of damping tensioner arm movement. The methodology taught in these patents, including the vibratory release that permits tensioner movement when necessary under the influence of engine vibration, has proven to be an extremely effective means of providing the desired tensioning function of a timing belt.
It is also known in the belt tensioner art to provide a belt tensioner with a fixed anti-kickback stop structure, which is used to prevent kickback or sudden movement of the tensioner in a direction against the belt-tensioning direction in response to sudden tightening of the belt (e.g. during rough periods of engine operation). The use of such fixed anti-kickback stop requires careful adjustment of the tensioner during installation, by advancing the tensioner toward the belt by rotating an eccentric internal mounting structure or other known method, so that the tensioner arm will assume a predetermined relative position with respect to the anti-kickback stop when the tensioner is installed. This is important so that the tensioner does not engage the stop prematurely during engine operation, which may cause over tightening of the belt, and also does not permit excessive kickback movement of the tensioner away from the belt, which may permit the belt to skip a tooth on an associated pulley about which the belt is trained. Such precise installation adjustment of the tensioner is tedious and time consuming. It typically requires a pointer associated with the arm to be aligned with some other indicia associated with the tensioner or engine. Even after this initial alignment is made, it may require re-adjusting after the engine is initially rotated after installation to remove any tolerance or xe2x80x9cstictionxe2x80x9d that may exist in the belt system.
Commonly assigned U.S. Pat. No. 4,583,962 discloses a timing belt tensioning device that includes a one-way clutch mechanism that permits pivotal movement in one direction and limits the pivotal movement in an opposite direction beyond a predetermined distance so as to prevent loosening of the timing belt to an extent that would allow tooth skip. The ""962 patent teaches a damping system that dampens the operating movements of the arm permitted by the one-way clutch mechanism. This construction eliminates the need for a fixed anti-kickback stop for various applications. However, the operation of the tensioner disclosed in the ""962 patent is such that the one-way clutch operated to prevent the tensioner arm from being substantially moved in a direction against the belt tensioning direction once the tensioner had moved toward the belt to take up belt slack. For certain applications, it may be desirable to enable the arm to back slowly away from the belt-tensioning direction to alleviate high tension in the belt.
It is an object of the present invention to provide a belt tensioner that avoids the installation difficulties presented by a fixed anti-kickback stop. It is a further object of the invention to provide a belt tensioner that will self adjust the arm stop position based upon the working position of the tensioner arm. It is also an object of the present invention to provide such a tensioner that has a one-way clutch mechanism that will prevent kickback of the tensioner greater than a threshold amount, and will permit the tensioner to be moved slowly against the belt tensioning direction when the belt is tightened (e.g., due to engine expansion under the heat of operation) to prevent over-tightening of the belt.
Other objects and advantages of the invention will become apparent from the following detailed description and appended drawings.